[meteorite-list] Thin Air - Cassini Finds Ethereal Atmosphere at Rhea

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 29 Nov 2010 12:17:59 -0800 (PST)
Message-ID: <201011292017.oATKHxrt024756_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2010-399

Thin Air - Cassini Finds Ethereal Atmosphere at Rhea
Jet Propulsion Laboratory
November 29, 2010

NASA's Cassini spacecraft has detected a very tenuous atmosphere known
as an exosphere, infused with oxygen and carbon dioxide around Saturn's
icy moon Rhea. This is the first time a spacecraft has directly captured
molecules of an oxygen atmosphere - albeit a very thin one -- at a world
other than Earth.

The oxygen appears to arise when Saturn's magnetic field rotates over
Rhea. Energetic particles trapped in the planet's magnetic field pepper
the moon's water-ice surface. They cause chemical reactions that
decompose the surface and release oxygen. The source of the carbon
dioxide is less certain.

Oxygen at Rhea's surface is estimated to be about 5 trillion times less
dense than what we have at Earth. But the new results show that surface
decomposition could contribute abundant molecules of oxygen, leading to
surface densities roughly 100 times greater than the exospheres of
either Earth's moon or Mercury. The formation of oxygen and carbon
dioxide could possibly drive complex chemistry on the surfaces of many
icy bodies in the universe.

"The new results suggest that active, complex chemistry involving oxygen
may be quite common throughout the solar system and even our universe,"
said lead author Ben Teolis, a Cassini team scientist based at Southwest
Research Institute in San Antonio. "Such chemistry could be a
prerequisite for life. All evidence from Cassini indicates that Rhea is
too cold and devoid of the liquid water necessary for life as we know it."

Releasing oxygen through surface irradiation could help generate
conditions favorable for life at an icy body other than Rhea that has
liquid water under the surface, Teolis said. If the oxygen and carbon
dioxide from the surface could somehow get transported down to a
sub-surface ocean, that would provide a much more hospitable environment
for more complex compounds and life to form. Scientists are keen to
investigate whether life on icy moons with an ocean is possible, though
they have not yet detected it.

The tenuous atmosphere with oxygen and carbon dioxide makes Rhea,
Saturn's second largest moon, unique in the Saturnian system. Titan has
a thick nitrogen-methane atmosphere, but very little carbon dioxide and
oxygen.

"Rhea is turning out to be much more interesting than we had imagined,"
said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion
Laboratory, Pasadena, Calif. "The Cassini finding highlights the rich
diversity of Saturn's moons and gives us clues on how they formed and
evolved."

Scientists had suspected Rhea could have a thin atmosphere with oxygen
and carbon dioxide, based on remote observations of Jupiter's icy moons
by NASA's Galileo spacecraft and Hubble Space Telescope. Other Cassini
observations detected oxygen escaping from icy Saturn ring particles
after ultraviolet bombardment. But Cassini was able to detect oxygen and
carbon dioxide in the exosphere directly because of how close it flew to
Rhea - 101 kilometers, or 63 miles - and its special suite of instruments.

In the new study, scientists combined data from Cassini's ion and
neutral mass spectrometer and the Cassini plasma spectrometer during
flybys on Nov. 26, 2005, Aug. 30, 2007, and March 2, 2010. The ion and
neutral mass spectrometer "tasted" peak densities of oxygen of around 50
billion molecules per cubic meter (1 billion molecules per cubic foot).
It detected peak densities of carbon dioxide of around 20 billion
molecules per cubic meter (about 600 million molecules per cubic foot).

The plasma spectrometer saw clear signatures of flowing streams of
positive and negative ions, with masses that corresponded to ions of
oxygen and carbon dioxide.

"How exactly the carbon dioxide is released is still a puzzle," said
co-author Geraint Jones, a Cassini team scientist based at University
College London in the U.K. "But with Cassini's diverse suite of
instruments observing Rhea from afar, as well as sniffing the gas
surrounding it, we hope to solve the puzzle."

The carbon dioxide may be the result of "dry ice" trapped from the
primordial solar nebula, as is the case with comets, or it may be due to
similar irradiation processes operating on the organic molecules trapped
in the water ice of Rhea. The carbon dioxide could also come from
carbon-rich materials deposited by tiny meteors that bombarded Rhea's
surface.

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency, and the Italian Space Agency. NASA's Jet
Propulsion Laboratory, Pasadena, Calif., a division of the California
Institute of Technology in Pasadena, manages the mission for NASA's
Science Mission Directorate, Washington, D.C. The Cassini orbiter was
designed, developed and assembled at JPL. The ion and neutral mass
spectrometer team and the Cassini plasma spectrometer team are based at
Southwest Research Institute, San Antonio.

For more information about the Cassini mission, visit:
http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov .

Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jia-rui.c.cook at jpl.nasa.gov

2010-399
Received on Mon 29 Nov 2010 03:17:59 PM PST